Water-soluble polymers are long known to be very useful in both agricultural and industrial applications. Among the polymers that are extremely well known, belongs the class of water-soluble polymers known as linear polyacrylamides. They are very useful because of their superior properties of infinite solubility and low use rates, among others. By far, the major use for linear polyacrylamides is in the treatment of water, especially wastewater.
In wastewater treatment and uses like canal sealing, polyacrylamide flocculates, or agglomerates to form particles. This flocculation, where light particles are attracted together to form heavier particles, causes them to sink rather than float, clarifying the water containing the particles. In agricultural sprays, the ability of these polymers to retain water, control movement and add viscosity is utilized to enhance the effectiveness of the sprays, in addition to other purposes.
These linear polyacrylamides are commonly available in three commercial forms. First, as a water-dispersible solid, polyacrylamides are used in applications such hydroseeding and canal sealing. This form slowly dissolves in water, but has a tendency to agglomerate when added too quickly or all-at-once to water. When this happens, the lumps that are formed take hours, days or weeks to dissolve in water. This slow dissolution property is used to advantage in the applications specified, but is a highly undesirable trait in situations that require quick dispersion of the polymer.
Second, water-dispersed polymers are used, which have the distinct advantage of already being dissolved. It is used in agricultural spray deposition aids, among others. However, not much polymer can be dissolved in water, only up to about two (2%) percent, before the solution becomes too viscous to be handled easily. This material also generates a lot of expense due to the shipping of mostly water, thus creating great inefficiency.
The third historical form is an emulsion, more specifically a water-in-oil emulsion. This is a polyacrylamide, with the polyacrylamide chains contained in small droplets of water, that are dispersed in an oil, by using emulsifiers to help make the two phases mix. Emulsions are droplets or “bubbles” of liquid, known to those practiced in the art as “micelles”, suspended in another liquid with which the first liquid will not mix. The micelles are often called the “discontinuous phase” and the suspending liquid is called the “continuous phase”.
In the case of polyacrylamide emulsions, the polymer, polyacrylamide, is dissolved in the discontinuous phase, in this case the water phase; and the continuous phase is oil. This is known as a water-in-oil (W/O) emulsion or a reverse emulsion. This type of emulsion keeps the polymer in small packets of water, which burst open when the emulsion comes in contact with water. Polyacrylamide emulsions disperse well in water with vigorous stirring and are used prevalently in water treatment.
Such emulsions are also used, among other things, in pesticide tank mixtures to aid in preventing drift and increasing deposition on target species. The problems with emulsions are that they form clumps, like solid forms, when the emulsion is added to water with little or no agitation or if the water to emulsion ratio is too low. Also, emulsions are inherently unstable and will eventually break or separate into oil and water layers. The oil rises as a layer, and the water layer sinks. Then since the polymer chains are now free to combine, because they are not separated by the oil “walls” (that is the oil and water separation or dividing line), they combine and form large lumps.
The polyacrylamide polymer itself comes in several types, defined by electrical charge of the polymer chain. It can be nonionic, anionic or cationic. The cationic form is commonly used in water treatment. In the agricultural applications, the cationic, or positively charged polymer, is rarely used, as it has a deleterious effect on aquatic wildlife. The nonionic or uncharged form is a reaction product of pure acrylamide, forming an uncharged, but water-soluble polymer that is quite inert in the environment.
Acrylamide is co-reacted with other monomers to form the cationic or anionic forms. To form the anionic polymer, acrylamide is most often reacted with an acrylate monomer that is further reacted so that it becomes negatively charged. The nonionic and anionic polymers have different properties. At lower levels in water, the anionic polymers build properties such as viscosity faster that are more desirable. Anionics are compatible with other charged molecules such as are contained in fertilizers. However, they can react undesirably with certain other charged molecules. Thus, nonionic polyacrylamides are used in situations where the anionics are incompatible with other molecules.
The amount of charge is measured as a percent of the comonomer added. Thus, a polyacrylamide that is 30% acrylate and 70% acrylamide is called a 30 percent-charged polymer. This percentage may be expressed as weight or mole percent, depending on the manufacturer. Typically, if the polymer is a combination of the two monomers, the acrylic acid portion is reacted with base to form the acid salt. The polymer is then considered to be charged.
Microemulsions are a very recent, commercially available development. A microemulsion is a special type of emulsion. These microemulsions have the same basic structure as traditional emulsions, except that the droplets are smaller. Smaller droplets, by virtue of the solution physics involved, are very stable and the droplets do not combine or separate in solutions as traditional emulsions do. Microemulsions are also virtually clear, while sometimes having only a slight haze, as opposed to a standard emulsion, which is milky white.
Microemulsions, as they are now, do have their own drawbacks, however. The biggest drawback to a microemulsion is that, if it is combined with water or aqueous solutions, microemulsions will form a skin at its surface that drastically reduces the water diffusion; and the diffusion of oil or emulsifier combination into the water phase. This is due to the fact that there are very many small aqueous droplets near the surface of the emulsion and when they are combined with water, water diffuses quickly across the discontinuous phase and swells the micelles nearest the surface. The micelles swell, combine, burst and rupture, in that order.
This instantaneous bursting of many of the droplets entangles the polymer on the surface of the microemulsion and forms a barrier, which, in turn, slows diffusion of water further into the microemulsion and dispersion of the rest of the polymer. This phenomenon, sometimes known as a “skin”, causes the same problems that traditional emulsions have in terms of dispersion and clean out.
Observers of microemulsions may actually observe that they are clear and therefore question the ability of the product to do the job intended or the presence, in this case, of polymer until the product is added to water and the characteristic milky appearance and slimy feel of polyacrylamide emulsion added to water appears.
While each of the polymers and the delivery systems have distinct advantages, certain applications create great disadvantage for all polymers. For instance, in fields that are watered using pivot irrigation, the polymer is known to have been tested and shown to be effective at reducing the need for water. However, handling of the traditional emulsion, which is, thus far, the only economical form for this application, can plug pumps, nozzles, screens, or other apparatus, when the clean out procedures are not followed properly because of the lumping process described above. Microemulsions are tested in this process and found to have the same problems because of the skinning process described. The current invention addresses many of these problems.